The present telephone network is an interconnected multi-node switching system, with each node serving a plurality of subscribers via twisted pairs of wire. The further the subscriber is from the switching node, the coarser the average aggregate gauge of his loop wire. Loops beyond a certain length having ohmic resistance larger than a certain maximum value, if they exist within the serviced area, must, in order to maintain the maximum permissible loss, utilize amplifying repeaters. In the recent past, the loop length beyond which repeating becomes necessary has been steadily declining due to the trend toward fine gauge wires, and possibly ultimately toward a unified fine gauge or "unigauge". The reasons, of course, are rising copper prices and declining cost of electronics.
If repeaters are used in the switching centres for longer loops, then it is most convenient for the operating companies to have a single standard repeater that self-adjusts its gain to fit the length of its loop. Two recent United States patents are directed to such self-adjusting repeaters. One is U.S. Pat. No. 3,903,378 issued Sept. 2, 1975 to David Q. Lee et al. It teaches the use of field effect transistors (FETs) as feedback elements to control the gain of associated amplifying devices in response to a voltage that is proportional to the loop length. The disadvantages of such simple arrangement is the lack of temperature stability of, and possibly the necessity of selecting, identical field effect transistors.
The second patent is U.S. Pat. No. 3,914,560 issued Oct. 21, 1975 to Clifford E. Greene, which patent teaches the use of the dynamic impedance of a d-c biased diode as the gain controlling element. Among the disadvantages of such arrangement are:
THE A-C SIGNAL NO MATTER HOW SMALL STILL MODULATES THE IMPEDANCE OF THE DIODE CAUSING HARMONIC DISTORTION;
IN ORDER TO MINIMIZE THE ABOVE DISADVANTAGE, THE VF signals are attenuated 40 to 50 dB before they are controlled to be reamplified thereafter, thus requiring low-noise high-gain amplifiers which are not inexpensive; and
DIODE IMPEDANCE VARIES CONSIDERABLY WITH TEMPERATURE AND IS DIFFICULT TO COMPENSATE.
The present invention endeavours to solve some of the above problems.